In extracorporeal circuit therapy where blood is taken out from the body of a patient, treated outside the body using a blood treatment apparatus, and put back into the body of the patient after treatment, a pressure measurement unit is typically used for measuring a pressure inside an extracorporeal circuit.
Japanese laid-open patent publication No. 09-024026 discloses, as an example of a means for measuring a pressure inside the extracorporeal circuit in a state in which a body fluid or a drug solution is prevented from making contact with the air, a pressure measurement unit which measures a pressure inside the extracorporeal circuit via a diaphragm.
FIG. 8 is a schematic configuration diagram showing an example of the configuration of this pressure measurement unit. A pressure measurement unit 1 is arranged in an extracorporeal circuit 8, the pressure measurement unit 1 including: an air chamber 9 having an air inlet-and-outlet port 50; a liquid chamber 6 having a liquid inlet port 40 and a liquid outlet port 41; a flexible diaphragm 20 which is sandwiched between the air chamber 9 and the liquid chamber 6 to provide a partition between the air chamber 9 and the liquid chamber 6, the flexible diaphragm 20 deforming in accordance with a pressure difference between a pressure inside the air chamber 9 and a pressure inside the liquid chamber 6; and a pressure measurement means 60 which is connected to the air inlet-and-outlet port 50 of the air chamber 9 via a communicating member 51, the pressure measurement means 60 measuring the pressure inside the liquid chamber 6 on the side of the air chamber 9 via the diaphragm. Since the flexible diaphragm 20 deforms in accordance with a change in the pressure inside the liquid chamber 6 and the pressure inside the air chamber 9 and the pressure inside the liquid chamber 6 change in a manner correlating with each other, the pressure measurement unit 1 measures the pressure inside the air chamber 9 and converts the measured value, thereby measuring the pressure inside the liquid chamber 6.
However, since the pressure measurement unit 1 shown in FIG. 8 is usually a disposable product, the pressure measurement unit 1 is configured to be detachable from the expensive pressure measurement means 60. Accordingly, if the pressure measurement means 60 and the pressure measurement unit 1 are detached from each other during use, it becomes impossible to measure the pressure in the extracorporeal circuit 8, and even they are connected to each other again, the initial position of the flexible diaphragm 20 immediately after the connection is not stable, and therefore, a pressure cannot be measured within a targeted pressure measurement range.
Gambro, Prismaflex Operator's manual, pages 193-199, discloses an example of a calibration method for solving such a problem. FIG. 9 is a schematic configuration diagram explaining the calibration method. A pressure measurement unit 1 includes: an air chamber 9 having an air inlet-and-outlet port 50; a liquid chamber 6 having a liquid inlet port 40 and a liquid outlet port 41; a flexible diaphragm 20 which is sandwiched between the air chamber 9 and the liquid chamber 6 to provide a partition between the air chamber 9 and the liquid chamber 6, the flexible diaphragm 20 deforming in accordance with a pressure difference between a pressure inside the air chamber 9 and a pressure inside the liquid chamber 6; and a pressure measurement means 60 which is connected to the air inlet-and-outlet port 50 of the air chamber 9 via a communicating member 51, the pressure measurement means 60 measuring the pressure inside the liquid chamber 6 on the side of the air chamber 9 via the flexible diaphragm 20. The communicating member 51 is configured to be detachable by a connection means 55. As shown in FIG. 9, a related art calibration system for the pressure measurement unit 1 includes the pressure measurement unit 1 arranged in an extracorporeal circuit 8; closing means 82 and 83 for closing the extracorporeal circuit 8 which are arranged on the upstream of the liquid inlet port 40 and on the downstream of the liquid outlet port 41, respectively; a sample port 84 which is arranged between the two closing means 82 and 83; and a syringe 85 which can be coupled with the sample port 84. Note that, in FIG. 9, components having the same functions as the components in FIG. 8 are indicated by the same reference numerals as those in FIG. 8.
In the pressure measurement unit 1 above, a calibration method which is carried out when the communicating member 51 in the connection means 55 is detached includes the following steps:                stopping a liquid sending means (not shown) in the extracorporeal circuit 8;        closing the liquid chamber 6 using the closing means 82 and 83;        inserting the syringe 85 into the sample port 84 and extracting 1 cc of body fluid or drug solution from the extracorporeal circuit 8 or infusing 1 cc of physiological saline into the extracorporeal circuit 8;        reconnecting the connection means 55 in the communicating member 51; and        opening the closing means 82 and 83.        
However, in the calibration method above, in step 2, the liquid chamber 6 is closed under the pressure of the extracorporeal circuit 8 as of the time when the liquid sending means is stopped, and therefore, for example, the liquid chamber 6 is closed by the closing means 82 with the flexible diaphragm 20 being deformed toward the liquid chamber 6 when the pressure inside the extracorporeal circuit 8 is negative, while the liquid chamber 6 is closed with the flexible diaphragm 20 being deformed toward the air chamber 9 when the pressure inside the extracorporeal circuit 8 is positive. Accordingly, the position of the flexible diaphragm 20 is not constant as it changes depending on the pressure inside the extracorporeal circuit 8 as of the time when the liquid chamber 6 is closed, and if the steps of step 3 onward are performed in such a state, the position of the flexible diaphragm 20 cannot be restored to a predetermined position at the time of starting the pressure measurement. Specifically, in the situation where the position of the flexible diaphragm 20 at the time of starting the pressure measurement has been moved toward the liquid chamber 6, the capacity of the liquid chamber 6 becomes small, and thus the measurement limit for negative pressures would be reduced and the range in which the pressure can be measured correctly would be narrowed. On the other hand, in the situation where the position of the flexible diaphragm 20 has been moved toward the air chamber 9, the capacity of the air chamber 9 becomes small, and thus the measurement limit for positive pressures would be reduced and the range in which the pressure can be measured correctly would be narrowed. Accordingly, there is a possibility that, even if the steps of step 3 onward are performed, the pressure cannot be measured correctly in a predetermined pressure measurement range. In addition, the related-art method has risks such as: a risk in which the body fluid or drug solution in the extracorporeal circuit 8 might be leaked outside the extracorporeal circuit 8 when the syringe 85 is inserted into or detached from the sample port 84; and a risk in which a needle (not shown) attached to an end of the syringe 85 might be accidentally stuck into a human body when it is inserted into the sample port, which might increase the risk of infection.